Abstract: Our review addresses one of the most used, but debated, topics in Ergonomics: Situation Awareness (SA). We examine and elaborate upon key SA models. These models are divided into individual SA, team SA and systems SA categories. Despite, or perhaps because of, the debates surrounding SA it remains an enduring theme for research and practice in the domain of Ergonomics, now for over two decades. A contingent approach, which seeks to match different models of SA to different types of ergonomics problem, enables the differences between positions to be revealed and reconciled, and the practitioner guided towards optimum methodological solutions.

Practitioner Summary: Measuring situation awareness (SA) in individuals, teams and systems has become a key objective in Ergonomics. One single approach to SA does not fit all problems encountered. This review shows the importance of considering all three types of models and achieving a match between them and the problem at hand.

THE State of the ScienceSituation awareness (SA) is one of the most keenly studied topics in Ergonomics (Wickens, 2008; Salmon and Stanton, 2013; Stanton et al, 2010). It is also one of the most hotly debated. SA is used to describe how people, and increasingly entire socio-technical systems, become and remain coupled to the dynamics of their operational environment (Moray, 2004). As a concept, SA provides researchers and practitioners with various models and methods to either describe what SA comprises, to determine how individuals, teams or systems develop SA, or to assess the quality of SA during task performance (Salmon and Stanton, 2013). Such accounts should also provide explanations for what happens when SA is ‘lost’, and how it affects performance when it is first gained or later regained (Stanton et al, 2015).
Unusually, for an Ergonomics concept, it has entered into wider society’s mainstream lexicon. SA is used in many settings and professions to refer interchangeably to information that resides in people’s heads and/or minds (Fracker 1991, Sarter and Woods 1991, Endsley 1995) or even brains (Endsley, 2015). It has more recently been viewed as something that exists in the world, in displays or other environmental features (e.g. Ackerman, 1998, 2005). It has also been expressed as an emergent property of people and their environment (Stanton et al. 2006, 2009a, 2010). SA has been explored in many operational contexts, including military settings (e.g. Endlsey 1993; Salmon et al, 2009; Stanton et al, 2006; Stanton, 2014; Stewart et al, 2008), transportation (e.g. Ma & Kaber, 2007; Golightly et al, 2010, 2015; Salmon et al, 2014, Walker et al, 2008, 2009) sport (Bourboussan et al, 2011; James & Patrick, 2004; Macquet & Stanton, 2014; Neville and Salmon, 2016), health care and medicine (Bleakley et al, 2013; Fioratou et al, 2010; Hazlehurst, McCullen & Gorman, 2007; Schulz, 2013), process control (Salmon et al, 2008; Sneddon et al, 2015; Stanton et al, 2009b) and emergency services (Seppanen et al, 2015; Blandford and Wong, 2004) to name a few. Experimental, theoretic and synthetic papers that deal with the concept of SA are among the most cited in the discipline (e.g. Lee, Cassano-Pinche´ & Vicente, 2005) and the term SA is one of Ergonomics’ most widely used (Patrick & Morgan, 2010). According to GoogleNgram (see Figure ) the term ‘situation awareness’ was hardly used at all within the corpus of literature prior to Endsley’s (1988), Woods (1988) and other pioneering publications on SA in the late 80’s. The line graph shown in Figure 1 accelerates dramatically from that point in time forward. Despite its prevalent use in many areas, our understanding of SA remains in constant development, even flux (Flach, 1995; Dekker, 2015). Indeed, for such a fast moving and important part of our discipline, a state-of-the-science review is not simply timely, it is arguably well over-due.
Figure – Google Ngram plot showing how the use of the term Situation Awareness in the English language lexicon has accelerated dramatically since the 1980s

Like many comparable concepts, such as workload (Young et al, 2015), trust (Kauer et al, 2015), mental models (Revell and Stanton, 2012), and safety (Hignett et al, 2013) there is no universally accepted definition of situation awareness. There are definitions that have proven to be more popular than others, but there are clear differences in how researchers and practitioners understand and define the concept. This has been starkly illustrated in recent journal special issues on SA (e.g. Pritchett, 2015) wherein the topic has proven to be unusual for the degree of contention it generates (see Carsten and Vanderhaegen, 2015; Endsley, 2015; Salmon et al, 2015; Stanton, 2010, 2016; Stanton et al, 2015). Providing that positions do not become excessively entrenched and overly dogmatic, for the profession, this debate creates a rich and exciting constellation of ideas and contentions around SA (Stanton, 2016; Salmon and Stanton, 2013).

Taken at face value, the discipline already has a dominant theory of SA (the three level model promulgated by Endsley in a Special Issue of the Human Factors journal in 1995 being undoubtedly the most popular). There are also associated methods with which to convert this model into ergonomics practice (of which there are many examples, see Salmon et al, 2006). If the discipline were to be defined purely as a pragmatic endeavour, and success judged solely in terms of practical application, then a state of science review of SA might well prove exceedingly short and not at all contentious. Thankfully, for our science, this is not the case. Practical concerns are key, but their value derives from the state of the underlying science. What makes SA so captivating is how radically different the concept of SA looks when projected through different theoretical foundations and associated world-views. What is becoming increasingly evident is that the worldview through which early models of SA were projected is quite different from alternate world-views that are emerging today. When Endsley’s first paper on SA was published in the 1988 Proceedings of the Human Factors and Ergonomics Society Annual meeting, the paradigm of experimental and cognitive psychology was firmly established and even deeply entrenched in the discipline. This dominance is something that van Winsen and Dekker (2015) have referred to as the ‘first cognitive revolution’. Ideas around systems thinking, specifically distributed cognition (e.g. Hutchins, 1995a, b) and cognitive systems engineering (Rassmussen et al, 1994) were far from mainstream at that time. In many cases, they were in their infancy. Compare this with today. There is a much stronger and growing systems focus (Salmon et al, 2015; Dul, et al., 2012) and increased recognition of the importance of systems concepts like complexity (Walker et al., 2010), constraints (Vicente, 1999), dynamism (Dekker & Woods, 2000), multiplicity (Lee, 2001), fuzziness (Karwowski 2000, Lee et al. 2003), randomness (Hancock, et al. 2000) and a myriad of allied terms, like resilience (Hoffman and Hancock, 2016). These are used to describe the kinds of problems Ergonomists are called upon to resolve. Nearly three decades since the term situation awareness entered Human Factors the standard paradigm has shifted, with the second cognitive (systems) revolution now being upon us (van Winsen & Dekker, 2015). One question we look to address here is: how has SA shifted with these emerging paradigm shifts?
The goal of this state-of-the-science review, therefore, is to convey the richness of the current debate around SA. The purpose of this review is NOT to dissect the state of empirical science; rather it draws on the empirical findings is to assess the current status of SA theory and models. It will evaluate the SA concept through different world-views, highlight where the main debates originate, how they can be reconciled, and present practical implications for the benefactors of the services Ergonomists dispense. We argue it is insufficient to judge any important Ergonomics concept purely on the criterion of practical expediency. We have to understand and explain the philosophical foundations and assumptions upon which our science is founded. Consequently, to present the state-of-the-science in this review effectively, we need to go beyond current practice and return to our fundamental foundations.

describing Situation AwarenessAt the most basic level, avoiding for the moment any strong links to a particular paradigm, situation awareness could be described simply as “knowing what is going on around you” or “having the big picture” (Jones, 2015, p. 98). To be more specific, situation awareness could be described as the ability of actors (usually humans, but not necessarily so) to become and remain coupled to the dynamics of their environment (e.g. Woods, 1988; Moray, 2004). This is on the assumption that being coupled to the dynamics of an environment is a desirable state of affairs for behaviours occurring within that environment (but again, this need not necessarily be so).
Knowing what is going on around you, as a high-level description of SA, imbues the concept with a strong ‘situation focus’, an objective ground-truth against which ‘awareness’ can be judged. There is an appealing cause (situation) and effect (awareness) logic in this ‘ground truth’ principle. It is only upon closer inspection that important subtleties are revealed. The notion of a ground-truth against which awareness of a situation can be judged tends to presume a “mapping of the relevant information in the situation onto a mental representation of that information within the [individual]” (Rousseau, Tremblay & Breton, 2004, p. 5). The term ‘mental representation’ reflects two further aspects of ‘awareness’. Firstly, that the ‘relevant information’ contained within a ‘mental representation’ is structured: SA is not merely about the presence or absence of discrete elements but also their contextual interconnections. Billings (1995) goes on to refer to an “abstraction within our minds” which reflects a second aspect of awareness: it is hypothetical in nature (Bryant et al., 2004). Awareness of a situation by an individual is not a canonical model with a direct one-to-one, homeomorphic, mapping to ‘reality’ (e.g. Banbury, Croft, Macken & Jones, 2004). Rather, it is “a representation that mirrors, duplicates, imitates or in some way illustrates a pattern of relationships observed in data or in nature […]”, “a characterisation of a process […]” (Reber, 1995, p.465) which may indeed contain isomorphic or polymorphic characteristics (but again, not necessarily so). It can be assumed that SA needs to provide individuals with “explanations for all attendant facts” (Reber, 1995; p. 793) in order for them to make better decisions and reap the benefit of improved performance.
A key issue is that explanations for attendant facts do not necessarily require either a particularly rich or detailed model of the situation. Indeed, it would be a highly inefficient form of representation if it did, and one not likely to have been supported by millennia of evolution. Rather, there is robust evidence that the better the mental representation (or awareness), the more parsimonious it is (e.g. Gobet, 1998; Chase & Simon, 1973). Information in ‘working memory’, upon which awareness is based (e.g. Bell & Lyon, 2000), can be forced to higher, more implicit levels of abstraction (Walker et al, 2009). The paradox is the better a person (or ‘agents’) ‘mental theory’ of their situation the less likely it is to have a direct one-to-one, isomorphic, mapping to the object or situation that is being perceived. In other words, ‘ground truth’ may have very little direct mapping with ‘awareness’. Furthermore, such awareness could conceivably represent one of many possible ‘ground truths’, each one individually capable of yielding optimal performance. It is well known that experts are able to chunk information (Chase and Simon, 1973) and that perception of the situation as a whole is greater than the sum of its parts (Duncan, 1979). In addition, there may be many paths to success, rather that just one right way (Vicente, 1999).
High-level definitions of SA also carry with them a linear or sequential flavour. This point is contentious (see Endsley, 2015; Stanton, Salmon & Walker, 2015) yet it is fair to say that many models of SA, including the most dominant (e.g. Endsley, 1995), assume that “humans typically operate in a closed-loop manner” (Endsley, 1995, p. 33). Whilst there can be little doubt that human performance does indeed rely on a chain of input-processing-output-feedback, there are contexts where this is not the case. Indeed, a larger proportion of behaviour than is often acknowledged is feed-forward (Plant & Stanton, 2013) where ‘processing’ and ‘outputs’ can conceivably occur without there being ‘input’ in the sense it is normally understood. Tasks such as driving, for example, which are routine and well learnt can often be performed on the basis of well-developed mental theories that require minimal input to guide effective behavior (Stanton et al, 2007; Walker et al., 2009c). Extreme examples of top-down, feed-forward behaviors are Driving Without Attention (May & Gale, 1998) and Mode Errors (Sarter and Woods, 1991), of which there are numerous examples, from rail (e.g. Stanton & Walker, 2009) to road (Stanton et al, 2011) to air (Griffin et al, 2010; Salmon et al, 2016). Situations like these result in part from top-down ‘knowledge in the head’ (awareness) overriding bottom-up ‘knowledge in the world’ (the actual situation: see Stanton & Walker, 2011). Expectations can play a major role in awareness of a situation and can be very misleading some of the time (Refferty et al, 2013). This is something mainstream models of SA readily acknowledge (see Endsley, 2015 for example) but in common usage around the wider discipline not always something that comes to the foreground. Instead, what tends to dominate the discourse (not exclusively, but commonly) is a focus on the presence or absence of specific features in a situation (e.g. Endsley, 1995; Fracker, 1991).
As well as linearity and sequentiality, high-level definitions of SA also carry a ‘normative’ flavour. The tacit assumption here being that the objective ‘ground-truth’ situation provides a relatively static and enduring reference point for judging the ‘goodness’ or ‘badness’ of SA (Endsley, 1988, p. 793). This is a generalization but one founded on a considerable body of visible work within the discipline. For example, see the reviews by Endsely (2015), Salmon et al (2006) and Wickens (2008). SA, however, is created for a purpose (Patrick & James, 2004) and that purpose, fundamentally, is to generate better situations – via improved performance - to subsequently be aware of. Smith and Hancock (1995) describe SA as “a generative process of knowledge creation” (p. 142) in which “[…] the environment informs the agent, modifying its knowledge. Knowledge directs the agent’s activity in the environment. That activity samples and perhaps anticipates or alters the environment, which in turn informs the agent” (Smith & Hancock, 1995, p. 142). Like many models of SA (e.g. Endsley, 1988) this process is cyclical in nature. Like all perception-action loops, the search for original ‘cause’ can be a diverting but often misleading one, something the Smith and Hancock (1995) model makes explicit by referring to SA as ‘constructive’ in its nature. The agent or actor is a part of the situation they find themselves and can influence its dynamics, indeed, are essential parts of its future. Thus, individuals are not always passive observers of a static normative situation, rather, they are agents in an interactive dynamic system, creating new ‘formative’ situations to become aware of (Stanton & Walker, 2011; Plant & Stanton, 2012). Clearly, SA is more complex then than the vernacular, everyday, use of the term suggests.

defining situation awarenessThe definition of SA has changed and developed over the past three decades, reflecting an evolution in foci of the Ergonomics discipline as well as the understanding of SA itself. Early definitions of SA used the term ‘situational awareness’ and focused on the individual person, as defined by Endsley:
“Situational awareness is the perception of the elements in the environment within a volume of time and space, the comprehension of their meaning and a projection of their status in the near future” (Endsley, 1988).
As research interest in SA grew, so it expanded from the individual level to the team level; a step from the personal to the social. Definitions of SA came to reflect this expansion, thus:
“the shared understanding of a situation among team members at one point in time” (Salas et al., 1995, p. 131)
Contemporary research into systems ergonomics has led to an interest in applying SA to larger and more complex interacting systems of human and technical agents. Again this has led to an evolution in definition, such as:
“activated knowledge for a specific task within a system….[and] the use of appropriate knowledge (held by individuals, captured by devices, etc.) which relates to the state of the environment and the changes as the situation develops” (Stanton et al, 2006, p. 1291).
In aviation, for example, one might study the awareness of individual pilots in the cockpit, specifically the pilot flying. Or SA analysis could be expanded to include the aircrew, not just the pilot flying but the captain, engineer/navigator, cabin and/or cargo crew. Subsequent analysis could also embrace the flight deck instruments, flight deck computers, flight director, autopilot, air traffic management system, flight data recorders, and so on (Salmon et al, 2016; Stanton et al, 2010; Sorensen et al, 2011). An expanded, but still legitimate comprehension of system SA can be extended out to the wider aeronautical transport system to include en-route and terminal controllers as well as overall systemic traffic-flow planning (Stanton, 2016; Stanton et al, 2015). After all, these collective agents (i.e. both human and non-human entities) also have to track and/or represent the situation in complex (and not so complex) ways in order to function as they should, or at the very least, help human agents to achieve acceptable levels of performance. This perspective expands even further into an overarching evaluation of the ‘system-of-systems’ (Harris and Stanton, 2010), which opens up even higher-level concerns for the global environment (e.g., whether ‘good’ SA at the individual level translates into good performance at the collective level, e.g. Walker & Manson2014.).
The systems-of-systems view represents a significant departure from the appealing cause and effect logic of individualistic models of SA rooted in cognitive psychology. The notion that SA resides in human as well as non-human (technology) agents is particularly controversial (Endsely, 2015). Consideration of human and non-human agent awareness is, however, becoming increasingly important given technological advances such as artificial intelligence and advanced automation (e.g. Lee, 2001; Hancock, 2014; 2016 ), and so too is the notion of multiple ‘agents’ cooperating over time in order to remain coupled to the dynamics of their environment (Stanton, 2016; ). Issues like these lie at the heart of wider Socio-Technical Systems (STS) approaches to overall system performance (Walker et al, 2009). The practical utility of these systemic strategies stem from their ability to provide insights into particularly thorny Ergonomics problems. Automation is a case in point. Take the recent disaster involving Air France 447 as an example (Salmon et al, 2016). Here the automated aircraft and cockpit systems were unaware of the current airspeed due to icing of the three pitot tubes (which measure air speed independently). Following the autopilot’s disconnection in response to this informational shortfall, the aircrew were unaware of why the autopilot had defaulted to this condition, what control inputs were needed to create a stable situation and what the flight status of the aircraft was (i.e. stall and rapid descent). Added to this is that each flight phase represented a set of different ‘situations’ for the system as a whole to be ‘aware’ of. Within each of these phases, the situation was changing dynamically for each individual agent. There can be no question the concept of situation awareness has much to offer in diagnosing incidents such as these, and from a variety of perspectives. It is possible to define the ground truth situation and the subsequent individual awareness, but it is also possible to examine team SA, or the SA of the entire system. Combining these perspectives, contingent on the insights that need to be extracted, means the system has to be studied at an appropriate level of analysis, be it individual, team or entire system-level comprehension. In what follows, we use this idea of levels of analysis to frame and evaluate current SA theoretical models.

Theoretical Models of SAThree distinct types of SA models have been identified: (i) individual, (ii) team and (iii) system. The theoretical bases for these models are now examined.
Models accounting for situation awareness held by individuals

Early models of SA focussed on individual operators and how they acquired SA cognitively during task performance (e.g. Adams, Tenney & Pew, 1995; Bedney & Miester, 1999; Endsley, 1995; Smith & Hancock, 1995). Heavily psychology-based, these models sought to explain the processes underpinning the awareness held in the minds of individual operators, that is, SA as experienced in the mind of the person (Stanton et al, 2010; Endsley, 2015). This focus incorporated the psychological processes involved (e.g. mental models, schema, perception, comprehension) and the nature of SA itself (e.g. the person’s SA comprising knowledge about x, y, and z). Such a focus explains why early discussion of SA so heavily featured the already well-rehearsed questions about the nature of human consciousness (James, 1890). Two of these models in particular stand out (Wickens, 2008): the three level model by Endsley (1995) and the perceptual cycle model by Smith and Hancock (1995). The former is by far the most popular, and has received increasing attention as individual models began to be integrated with wider and more comprehensive systems models (e.g. Stanton et al, 2009a, b). The latter represents a well-cited counterpoint founded on its own distinct theory-base. Both are shown, along with other relevant offerings, in Table .

Perception of elements, comprehension of meaning and projection of future status

Endsley (1988)

Three-level model

Started with aviation, later spread to other domains

2253

Situation awareness is based on the integration of knowledge resulting from recurrent situation assessments.

Sarter & Woods (1991)

Working memory, mental models, situation assessment awareness

Aviation

273

SA is the pre-requisite state of knowledge for making adaptive decisions in situations involving uncertainty, i.e. a veridical model of reality.

Taylor (1990)

Theories of attention and cognition

Military, air traffic control and nuclear power

272

Situation awareness is adaptive, externally-directed consciousness that has its products knowledge about a dynamic task environment and directed action within that environment.

Smith & Hancock (1995)

Perceptual cycle model

Air traffic control

204

The conscious dynamic reflection on the situation by an individual, providing dynamic orientation to the situation and an opportunity to reflect on the past, present and future and potential features of the situation.

Bedny and Meister (1999)

Theory of activity

Training, Information Systems and HCI

159

SA contributed to good performance, it is not synonymous with it. It is possible to have good SA but still not be a good pilot because of poor motor skills, coordination or attitude problems. Conversely, under automated flight conditions it is possible to have good performance with minimal SA.

Adams, Tenney & Pew (1995)

Perceptual cycle model,

working memory

Aviation

33

“…an abstraction that exists within our minds, describing phenomena that we observe in humans performing work in a rich and usually dynamic environment.”